1. Field of the Invention
This invention relates to an improved method and apparatus for determining the CO.sub.2 minimum miscibility pressure (MMP) of reservoir oil displaced by carbon dioxide.
2. Background of the Invention
A variety of supplemental recovery techniques have been employed in order to increase the recovery of oil from subterranean reservoirs. These techniques include thermal recovery methods, waterflooding and miscible flooding.
In miscible flooding, the use of carbon dioxide has received considerable attention in the industry because of its high displacement efficiency and relatively low cost. Miscible recovery of a reservoir oil can be achieved by CO.sub.2 displacement at a pressure level greater than a certain minimum, see Stalbergs, F. I., "Carbon Dioxide Miscible Flooding: Past, Present, and Outlook for the Future, " J. Pet. Tech. (Aug. 1978) 1102-1112. This minimum pressure is defined as the carbon dioxide minimum miscibility pressure (MMP).
The slim tube method has been used on a routine basis in the oil industry for many years for measuring the CO.sub.2 MMP of reservoir oil, see Yelling et al, "Determination and Prediction of CO.sub.2 Minimum Miscibility Pressure, " J. Pet. Tech. Vol. 32 (1), pg. 160 (1980).
In the slim tube method of measuring CO.sub.2 MMP, the apparatus consists of a long, sand-packed column of 50-200 foot length coiled in an oven. The sand is saturated with reservoir fluid at the reservoir pressure. Carbon dioxide is injected at one end at a constant back pressure to displace the oil. Fluids produced from the column are separated into liquid and gas by blow-down to atmospheric pressure. The liquid oil is collected and weighed and the gas volume is measured. At pressures above the MMP, the ultimate recovery of reservoir oil is close to 100 percent, whereas below the MMP, the ultimate recovery is much less than 100 percent. A plot of the ultimate oil recovery versus test pressure has a discontinuity at the MMP. A typical slim tube determination of MMP by measuring recoveries at four different pressures requires eight days of experimentation.
The present invention provides an improved method and apparatus for determining CO.sub.2 MMP that requires much less time than the slim tube test, e.g. one day compared to eight days, does not require handling any fluids during the experiment and the amount of reservoir oil required is about one-tenth that required for the slim tube.